The invention relates to an incandescent lamp comprising a filament formed by a plurality of filament segments, each segment consisting of a straight spiral section, said segments being mutually parallel and arranged in several layers.
A lamp as described in the opening paragraph is known from the document U.S. Pat No. 5,268,613. According to this document, a lamp placed in a reflector which is a body of revolution comprises a plurality of rectilinear helically coiled filament segments, arranged in two layers such that the filament segments are placed so as to be substantially equidistant to the axis of revolution of the reflector.
It is an object of the invention to provide better characteristics for the light beam, both in the case of a concentrated beam and in the case of a wide beam.
To achieve this, the straight filaments of a layer are disposed along a virtual involute curved surface within a layer of filament segments. The term xe2x80x9cinvolutexe2x80x9d is used in this specification to refer to a segment of an involute curve, rather than to the complete cross-section of an involute.
Special embodiments of the invention are defined in the dependent claims 2 to 8.
These as well as other, more detailed aspects of the invention will become clear from the following description of an embodiment, which is given by way of non-limitative example.